Getting a flu shot cuts the risk of having a heart attack or stroke by more than 50% in people who have had a heart attack, a new study shows.

"We may have identified that the flu vaccine may also be a vaccine against heart attacks," says lead author Jacob Udell, a cardiologist at Women's College Hospital and a scientist at the University of Toronto.

Udell and colleagues analyzed six studies on the heart health of more than 6,700 men and women, average age 67. Half got a flu vaccine; half got a placebo shot or nothing. About a third had heart disease and the rest had risk factors such as high blood pressure, high cholesterol, diabetes and smoking.

People who had had a recent heart attack who got the flu shot had a 55% lower risk of having a major cardiac event, such as a heart attack, stroke, heart failure or dying from another cardiac cause, Udell says. "The highest-risk patient got the highest reward for getting the flu shot."

Other findings, out Tuesday in JAMA, a journal of the American Medical Association, showed that people with a history of heart disease who had a flu shot:

• Had an overall 36% reduced risk of a major cardiac event, such as heart attack, stroke, heart failure or death from cardiac-related causes, one year later.

• Were less likely to die from cardiac-related or other causes. Most deaths were from cardiac causes, Udell says.

• Were 30% less likely to have a major cardiac event with a more potent vaccine compared with a typical seasonal vaccine. The stronger vaccine or a booster shot offered more protection, he says.

So why does the flu shot help?

There are a couple theories, Udell says. One is the "vulnerable plaque theory" -- that inflammation caused by the flu "may turn a stable plaque into an unstable plaque and cause a cardiac event."

Plaque is the result of a buildup in the lining of the arteries of fatty substances, cholesterol, calcium and fibrin (a clotting material in the blood).

Another is the "vulnerable patient theory." That suggests that the side effects from the flu, such as coughing, low oxygen, low blood pressure, fast heart rate and possible pneumonia, may strain the heart and cause a cardiac event, he says.

Bottom line: This is another good reason to get a flu shot, he says.

All the major health organizations recommend that people with heart disease get the influenza vaccine, says cardiologist Mariell Jessup, president of the American Heart Association. "However, this kind of data underscores the risk of death from the flu and how it can be prevented."